Erythrocytes are the red cells of blood which serve the biological function of transporting respiratory gases. In nature, the walls of the red cells are membranes which contain many different kinds of proteins and lipid materials. Oxygen passes through the erythrocyte walls and is exchanged for carbon dioxide which the erythrocytes carry away from the tissue.
It has long been common in the practice of medicine to take blood from a donor and transfuse this into the blood circulatory system of a patient who is deficient in hemoglobin. There are, however, difficulties in the preparation of blood for transfusion and substantial difficulties in maintaining adequate reserves of whole blood and/or blood components for transfusion.
One difficulty is that the natural erythrocytes in the blood of animals and humans deteriorate relatively soon after the blood is drawn, and present regulations require that the blood must be used for human transfusion within 21 days after it is drawn. Another serious inconvenience is that the blood of the donor must be typed and transfusions generally made into subjects whose blood is of the same type as that of the donor. Both of these disadvantages are due to the presence of proteins which are contained within the membranes of the natural erythrocytes.
U.S. Pat. No. 4,133,874 discloses a process in which a lipid in an organic solvent is spun to form a film on the interior walls of a container, and this film allowed to dry. Stroma-free hemoglobin is added, and by the use of ultrasound, hemoglobin is encapsulated within lipid composition membranes to form synthetic erythrocytes.
The '874 patent teaches that synthetic erythrocytes having hemoglobin solution encapsulated in lipid composition membranes can be used to transport respiratory gases in warm blooded animals; however, one with knowledge of the function of erythrocytes would recognize that improvements over the preparations described in the '874 patent might greatly enhance their utility.
The sonification method in the '874 patent is useful for producing synthetic erythrocytes under laboratory conditions but is not readily adaptable to mass production techniques. Importantly, the sonification method presents obstacles to providing and maintaining sterility of the preparation.
The most concentrated hemoglobin fraction encapsulated in the disclosure of the '874 patent is 22 gram percent hemoglobin, i.e., about two-thirds the concentration of hemoglobin within the erythrocytes of healthy humans. Using this concentration of hemoglobin, the preparation at a 50 percent hematocrit (slightly greater than normal whole blood) is necessarily less than 12 percent, and accounting for the synthetic erythrocyte membranes and the void volume between packed cells, the total hemoglobin at 50 percent hematocrit would not be more than about 9 gram percent. This compares quite unfavorably with the total hemoglobin of about 15 gram percent found in normal human blood.
The synthetic erythrocytes formed by the sonification process described in the '874 patent have a range of diameters of from about 0.1 microns to about 10 microns. The upper end of this size range is generally unsuitable for transfusion into warm blooded animals, being too large to fit through capillaries (human erythrocytes have a diameter of about 7 microns). Synthetic erythrocytes should be somewhat smaller than natural erythrocytes because synthetic erythrocytes are less flexible and do not pass as easily through the constricted capillaries. Several advantages accrue by providing synthetic erythrocytes within a narrow size range at the lower end of the size range described in the '874 patent.
An essential attribute of a synthetic erythrocyte preparation for transfusion into animals, and particularly humans, is that the preparation be sterile. The introduction of a synthetic erythrocyte preparation represents a dilution of infection-resistant agents normally present within blood, including antibodies produced by lymphocytes. In any case, a synthetic erythrocyte preparation should not introduce infectious agents. Synthetic erythrocyte preparations cannot be sterilized by heat or any other sterilization method which would denature the hemoglobin or destabilize the synthetic erythrocyte membranes.
An important projected use of synthetic erythrocytes preparations is to substitute for whole blood in remote locations where there is no readily available source of fresh blood. Whereas the shelf life of blood is about 21 days under refrigeration, synthetic erythrocyte preparations may be stored for considerably longer periods. It would be desirable to have synthetic erythrocytes which may be stored substantially indefinitely even when not refrigerated.